1.1.1 Genome shuffling of Saccharomyces cerevisae for multiple-stress resistant yeast to produce bioethanol

In the fermentation process, sugars are transformed into ethanol by addition of microoganism. Ethanol production from sugars has been commercially dominated by the yeast S. cereviseae (Tanaka, 2006). Practically, yeast cells are often exposed in multiple stress environments. Therefore, it is helpful to fermentation efficiency and economic benefits to breed the yeast strains with tolerance against the multiple-stress such as temperature, ethanol, osmotic pressure, and so on (Cakar et al., 2005). Yeast strain improvement strategies are numerous and often complementary to each other, a summary of the main technologies is shown in Table 2. The choice among them is based on three factors: (1) the genetic nature of traits (monogenic or polygenic), (2) the knowledge of the genes involved (rational or blind approaches) (3) the aim of the genetic manipulation (Giudici et al., 2005; Gasch et al., 2000 ).

It is difficult to improve the multi-tolerance of the yeast by rational genetic engineering technology before its mechanism completely clarified. Nevertheless, for quantitative traits, the number of responsible genes QTLs is so great that a "gene-by-gene" engineering strategy is impossible to perform. In these cases, blind strategies, such as genome shuffling (Zhang et al., 2002), could be applied in order to obtain quickly strains with recombinant traits. Genome shuffling is an accelerated evolutionary approach that, on the base of the recursive multiparental protoplast fusion, permits obtaining the desired complex phenotype more rapidly than the normal breeding methods (Figure 6). Genome shuffling technology can bring a rapidly improvement of breeding a hybrid with whole-genome random reorganization. After the initial strains in various long term evolution experiments (Figure 7), we successfully applied the genome shuffling technology that combines the advantage of multi — parental recursive fusion with the recombination of entire genomes normally associated with conventional mutant breeding to selecting the multiple-stress resistant yeast (Figure 8).